1. Field of the Invention
The present invention relates a broadband wireless access system and, more particularly, to a method for a mobile station and a base station to perform an efficient ranging procedure considering a ranging purpose and a device for performing the same.
2. Discussion of the Related Art
The main standard established by the IEEE 802.16 Working Group includes the IEEE 802.16-2004, which is referred to as a Fixed WiMAX, and the IEEE 802.16e-2005 (hereinafter referred to as 16e), which is referred to a mobile WiMAX. The IEEE gave its final approval for the IEEE 802.16e-2005 on December 2005. The most current version of the mobile WiMAX technology is based upon standards including the IEEE 802.16-2004, the IEEE 802.16e-2005 (this document is included in the Corrigenda of the IEEE 802.16-2004), and the IEEE 802.16-2004/Corrigenda/D8. Currently, a standardization procedure of the IEEE 802.16m for a next generation mobile WiMAX is in process by TGm within the IEEE 802.16 Working Group.
Among the process steps for performing a first (or initial) net registration procedure, a step of adjusting transmission parameters (i.e., frequency offset, time offset, transmission power) performed by the mobile station in order to perform an uplink communication with the base station is referred to as “initial ranging”. And, after performing the net registration procedure, the mobile station performs periodic ranging in order to continuously maintain the uplink communication with the base station. Additionally, other ranging types include handover ranging enabling the mobile station to simplify procedures during handover operation, and bandwidth-request ranging enabling the mobile station to request for an uplink bandwidth when data that are to be transmitted have been generated.
In a broadband wireless access system, when performing a ranging process in accordance with each ranging type, a CDMA code (or ranging preamble) group and a region to which the CDMA code is to be transmitted are allocated (or assigned) by the network through a channel (e.g., UL-MAP) broadcasting system information. Accordingly, for example, in order for the mobile station to perform handover ranging, the mobile station should request for handover ranging, by selecting a specific code among the CDMA codes for handover ranging, and by transmitting the selected code to the network through initial raging and handover ranging regions. By using this method, the network may be capable of identifying different ranging types through the received CDMA code and the section to which the CDMA code is transmitted.
More specifically, in order to perform initial ranging, the mobile station of the IEEE 802.16m standard (AMS) acquires downlink synchronization with the base station, wherein the base station becomes the target for performing the ranging process, and then receives an uplink transmission parameter. Depending upon a random back-off, the mobile station selects a ranging slot (or ranging opportunity) and randomly selects an initial ranging preamble code, thereby transmitting the randomly selected initial ranging preamble code to the selected ranging slot. The base station receiving the ranging preamble code transmits a ranging acknowledgement (AAI_RNG-ACK) message to the mobile station. Herein, the ranging acknowledgement message may include a ranging preamble code transmitted by one or more mobile stations in a predetermined frame, reception state of the code, and a physical compensation value based upon the reception status.
More specifically, when the base station receives ranging preamble codes from the mobile station, as a response to the received ranging preamble codes, the base station transmits allocation (or assignment) information of the AAI_RNG-ACK message using a Broadcast Masking Code to the mobile station through a broadcast assignment A-MAP IE. When an AAI_RNG-ACK message is transmitted to a specific mobile station by non-request, allocation (or assignment) information is transmitted to the mobile station through a DL basic assignment A-MAP IE using an unicast STID.
When the mobile station receives a ranging acknowledgment message and verifies the ranging acknowledgement bitmap of the corresponding message, in case there are no successively decoded ranging preamble code existing in the ranging slot selected by the mobile station, or in case there are no response to any initial ranging attempts made by the mobile station included in the ranging acknowledgement message, the mobile station may determine that the initial ranging processes attempted by the mobile station itself has failed. Accordingly, the mobile station restarts the initial ranging procedure.
In case the initial ranging is successfully received, the base station may provide the following three difference types of responses to the mobile station through the AAI_RNG-ACK message. First of all, in case the ranging status is ‘continue’, a physical compensation value is included in the corresponding message. Secondly, in case the ranging status is ‘success’, the base station assigns (or allocates) uplink resource for transmitting a ranging request message. At this point, a physical compensation value may be further included. Finally, in case the ranging status is ‘abort’, the base station may request the mobile station to stop the ranging procedure for a predetermined period of time.
In case the ranging status is ‘continue’, the mobile station performs a quick ranging procedure using the physical compensation value included in the AAI_RNG-ACK message. And, in case the ranging status is ‘success’, the mobile station is assigned with an uplink resource from the base station and transmits an AAI_RNG-REQ message through the assigned uplink resource.
The AAI_RNG-ACK message corresponds to a response to initial ranging as well as handover/periodic ranging. The base station transmits the corresponding message within a pre-defined time point (i.e., section: transmission offset).
In case of the IEEE 802.16m system, the handover procedure should be completed within a handover interruption time (30 ms) upon request.
Therefore, since the operations belonging to the handover procedure should be performed within minimum delay, as one of the operations that can be performed during the handover process, the base station should quickly transmit an AAI_RNG-ACK message to the mobile station as a response to the handover ranging, so that the mobile station can quickly acknowledge whether or not the handover ranging preamble code transmitted by the use equipment itself has been received. At this point, in light of the overhead, it may be inefficient to define the transmission point of an AAI_RNG-ACK message for the initial/periodic ranging to have the same transmission point of the AAI_RNG-ACK message for the handover ranging. Accordingly, the transmission point and procedure of differentiated AAI_RNG-ACK messages should be defined in based upon each ranging purpose.